Problem: Solve for $q$ : $13 + q = 6$
Subtract $13$ from both sides: $ \begin{eqnarray} \\ 13 + q &=& 6 \\ \\ {-13} && {-13} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{13 + q} &\hphantom{=}& \hphantom{6} \\ q &=& 6 {- 13} \\ \end{eqnarray} $ Simplify. $ q = -7$